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Full text of "Distribution, reproduction, and growth of Manila clam, Tapes philippinarum, in British Columbia"

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Journal of shellfish research. 

[S.I. :National Shellfisheries Association,! 981- 
http://www.biodiversitylibrary.org/bibliography/2179 



V. 2 (1982): http://www.biodiversitylibrary.org/item/18781 
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Journal of Shell fish Research, Vol. 2, No. 1, 47-54, 1982. 



DISTRIBUTION, REPRODUCTION, AND GROWTH OF MANILA CLAM, TAPES PHILIPPINARUM 

(ADAMS AND REEVES), IN BRITISH COLUMBIA 



NEIL BOURNE 

Department of Fisheries and Oceans 

Fisheries R esearch Branch 

Pacific Biological Station 

Nanaimo, British Cohimbia, Canada V9R 5K6 



ABSTRACT Manila clams {Tapes philippinarum) were first found in British Columbia in Ladysmith Harbour in 1936. In 
30 years they spread throughout the Strait of Georgia and along the entire western coast of Vancouver Island to become 
one of the major intertidal bivalves. In the 1960's, Manila clams spread to the Queen Charlotte Strait area and, in the 1970*s, 
to the central coastal area. Probable dispersal routes to these areas are discussed. Two intentional transplants to establish 
breeding populations in the northern part of the Province were not successful. Growth rates of Manila clams are fastest in 
the Strait of Georgia where water temperatures are highest, and slowest in Queen Charlotte Strait where water temperatures 
are lowest. Further northward dispersal of this species along the British Columbia coast is discussed. 



INTRODUCTION 



Manila clams {Tapes philippinarum) are indigenous to 
Japan between latitudes 25 to 45 N where they are widely 
used in the commercial fishery (Tamara 1966). The species 
was accidentally introduced into British Columbia presum- 
ably with seed of the Pacific oyster Crassostrea gigas 

(Thunberg) from Japan, and spread rapidly throughout the Nanaimo about 25 km north of Ladysmith Harbour. In 

southern part of the Province (Quayle 1964). Manila clams 1941, they had entered the commercial catch (Quayle and 

soon found general acceptance as an edible mollusc in Bourne 1972). 



DISPERSAL IN THE STRAIT OF GEORGIA 

In British Columbia, Manila clams were first found in 
Ladysmith Harbour (lat. 49°N) in 1936. They spread 
rapidly throughout the Strait of Georgia (Quayle 1960, 
1964) (Figure 2) and, by 1942, Neave (1949) reported 
they were the most common bivalve in Departure Bay at 



British Columbia and now support commercial and recrea- 



Although the Strait of Georgia is north of the traditional 



tional fisheries (Quayle and Bourne 1972) (Figure 1), the latitudes where Manila clams occur in Japan, water tempera- 
accidental introduction is considered beneficial. Discovery tures in the Strait are favorable for Manila clam reproduction, 
and initial dispersal of this species in southern British Holland and Chew (1974) and Ohba (1959) reported the 



Columbia were previously reported (Neave 1949, Quayle 
1964, Quayle and Bourne 1972, Bourne 1979). Tlie present 



400- 



presence of ripe gonads and subsequent spawning in Manila 
clams when water temperatures exceeded 14°C. Mann 
paper provides recent information on the continuing spread (1979) found gonadal development slow at 12 C and no 
of this bivalve, routes of dispersal, reproduction, and spawning until temperatures reached 15°C. In the Strait, 
compares growth rates in several areas in British Columbia. monthly mean surface water temperatures of 1 5 C and above 

are common during summer months (Hollister and Sandes 
1972), which is adequate for successful reproduction. 

Surface currents aided the spread of Manila clams in the 
Strait of Georgia because water circulation is open and no 
natural barriers are present to prevent spread of pelagic 
larvae. Waldichuk and Tabata (1955) and Waldichuk (1957) 
showed there is a general counterclockwise surface circula- 
tion in the Strait. Tully and Dodimead (1957) described 
the dominant feature of its tital circulation. On flood tides, 
water from the south is displaced northward, principally 
on the mainland side. The flood tide from the north is 
strongest on the Vancouver side and creates a southward 
flow. Thus, there appears to be a continuity of flow north- 
ward along the mainland side of the Strait, across the 
northern end and then southward along the Vancouver 
Island side. On ebb tides, the southward movement is 
negligible on the mainland side, but it is the dominant flow 
along the Vancouver Island side of the Strait. Such currents, 
along with wind-driven currents, would disperse pelagic 



300- 



o 

I- 

o 

UJ 

2 
100 




1951 



1955 



I960 1965 

YEAR 



1970 



1975 1980 



Figure 1. Annual commercial landings of Manila clams (whole weight 
in British Columbia, 1951-1980, 



larvae througliout the Strait. 



47 



48 



BOURNE 




Figure 2. Map of British Columbia showing locations mentioned in the text. 



Manila clams inhabit the mid-to-upper portion of the 



Surface water currents from Puget Sound do enter the 



intertidal beach, an ecological niche that apparently was southern part of the Strait of Georgia and could carry 
not dominated by any species prior to its introduction, pelagic larvae northward. The introduction and spread of 
and this contributed to the rapid spread of the species in Manila clams in British Columbia undoubtedly resulted 

from stock planted there and not from dispersal of clams in 
the Puget Sound area. Similarly it is unlikely there was a 
southward movement of Manila clams from British Columbia 
to Puget Sound; the present stock in Puget Sound 
undoubtedly developed from clams imported there. 

Manila clams did not spread northward of the Strait of 
Georgia because of the cold water temperature barrier at 
Seymour Narrows and the Yuculta Rapids (Figure 2). 
Although monthly mean surface water temperatures of 
14.3°C have been recorded at the southern end of Seymour 
Narrows (Hollister and Sandes 1972), water temperatures 
there and to the north are probably not sufficient to allow 
spawning or larval development. Water temperatures at 
the Narrows are probably colder than at the southern end; 



the Strait. Maximum tidal amplitude in the Strait is about 
4.9 m. Width of the Manila clam zone varies with slope of 
the beach; on steep-sloped beaches it may be only a few 
meters, and on a few wide beaches with gentle slopes, the 
width is about 75 m. Manila clams do not occur in the sub- 
tidal area in British Columbia. Heavy mortalities occasionally 
occur in abnormally cold winters (e.g., the winter of 1968— 
1969 [Bourne, unpublished] ) because the clams occupy 
upper beach levels and burrow to only shallow depths in 
the sediment (10 cm maximum). 

On the Pacific coast of North America, Manila clams 
were first found in British Columbia rather than in the 
State of Washington where earlier and heavier plantings of 
Pacific oyster seed from Japan occurred. Their presence was 



not recorded in Washington until the early 1940's. Manila a 1-year observation at Yuculta Rapids showed a maximum 
clams are now abundant in Puget Sound and are used in monthly mean surface water temperature of 9.9 C in July 
both the commercial and recreational fisheries (Goodwin (Hollister and Sandes 1972). No Manila clams have been 



1973,WUliams 1978). 



found during extensive surveys of beaches immediately 



Manila Clams in British Columbia 



49 



north of this area (Bourne, unpubHshed). Commercial of Georgia and along the western coast of Vancouver Island 



landings have been reported from some northern areas in 
the Fisheries Statistics, but these are believed to be in error. 
Dispersal of Manila clams southward, through Juan de 
Fuca Strait, did not occur, probably because of the cold 
water barrier. Maximum monthly mean water temperatures 
at three sampling stations in this area are all below 12°C 
(Hollister and Sandes 1972), too low to allow spawning and 
larval development. 



to become one of the major intertidal bivalves. 



intentional transplants 

Two attempts were made to transplant populations of 
Manila clams to the northern coast of British Columbia 
(Figure 2). In 1962, about 15,000 adults from Ladysmith 
Harbour were planted in Naden Harbour and a similar 
number were planted in Masset Inlet on the northern coast 
of the Queen Charlotte Islands (D. B. Quayle, personal 
communication) (Figure 2). In 1969, about 100 adults 
from Baynes Sound in the Strait of Georgia were introduced 
to Cosine Island, Principe Channel. 

Clams survived in both areas but growth was poor, no 
apparent reproduction occurred, and the populations 
eventually disappeared. The reasons these populations did 
not become established are unknown, but low water 
but Manila clams were abundant there in the early 1950's temperatures were probably a major factor. Maximum 
(D. B. Quayle, Pacific Biological Station, personal communi- monthly mean surface water temperatures of sliglitly above 
cation). Water temperatures in Barkley Sound are suitable 14 C have been recorded at Masset (Hollister and Sandes 
for Manila clam reproduction and the species spread rapidly 1972); Black and Elsey (1948) found surface water tempera- 
throughout the Sound, again occupying the mid-to-upper tures were generally below 14 C at the head of the Inlet 
portion of intertidal beaches. 



dispersal along the western COAST 

OF VANCOUVER ISLAND 

Manila clams occur along the entire western coast of 
Vancouver Island but dispersal there is less well documented 
than in the Strait of Georgia. They were probably introduced 
when Pacific seed oysters were planted in Barkley Sound 
(Figure 2); the exact date of the introduction is unknown 



Dispersal along the western coast of Vancouver Island 



(where the transplant occurred) although they have risen 
to 20 C briefly in August. No water temperature data are 



was rapid and, by the late 1950's, Manila clams were estab- available from Cosine Island but nearby at Bonilla and 

lished in Esperanza Inlet (Quayle 1960) about two-thirds Triple islands, monthly mean water temperatures rarely go 

northward along the western coast of the Island (Figure 2). above 1 S'^C. The time-temperature period for gametogenesis, 

Inshore currents along this coast have a northward direc- as described for C gigas and Ostrea edidis Linne (Mann 



1979), may have been too short at both localities to permit 
complete gonadal development. If the Naden Harbour 
population had spawned the larvae may have been flushed 
out into Dixon Entrance where low water temperatures 



tion (Tally 1937) and would carry larvae that drifted out 
of sounds northward along the coast. Monthly mean summer 
water temperatures along the outer coast range from 13 to 
16 C (Hollister and Sandes 1972) which is sufficient to 
to permit larval development, particularly in summers with could cause mortalities. The size of the introduction at 
above -average water temperatures. Yoshida (1953) stated Cosine Island may have been too small to permit establish- 
that the larval period of Manila clams at these temperatures ment of a population. If successful breeding had occurred 
would be 3 to 4 weeks and Williams (1978) reported a at either location, juveniles may have been killed because 



similar period in Puget Sound. In a 3- to 4-week period, 
larvae could drift out of a sound, be carried northward 
along the coast, enter another sound, settle, and establish a 
population. Water temperatures in local areas in sounds 
along the western coast of the Island are warmer than on 
the outer coast and are quite adequate for successful repro- 
duction. Manila clam populations in inlets along the western 
coast of Vancouver Island occupy the mid-to-upper part of 
the intertidal beach. 

It was expected that the spread of Manila clams might be 
halted at the Brooks Peninsula because it tends to be a 
biological barrier. Species such as the clam Rhamphidonta 



of low winter temperatures. 

dispersal TO OTHER AREAS 

In 1972, two adult Manila clams (47- and 48-mm shell 
length) were found at Spider Anchorage in the central 
coastal area of the Province (lat. 51*^5f'N;long. 128^^13 W) 
(Figure 2). Eleven more clams were found there in 1973, 
ranging in length and age from 45 to 60 mm and 7 to 9 
years, respectively, and apparently arrived there in the 
mid-1960*s (Bourne, unpublished). All clams were at the 
mid-intertidal beach position. Further extensive surveys of 
this area and areas to the north of it in 1973 failed to yield 



retifera Bernard occur south but not north of this peninsula any more Manila clams. 



(F. R. Bernard, Pacific Biological Station, personal communi- 



Extensive surveys in the Queen Charlotte Strait-Alert 



cation). However, in 1966, an extensive population of Bay area (Figure 2) in the 1960's and early 1970'sdidnot 



clams was found in Quatsino Sound, the most northerly 
major inlet on the western coast of Vancouver Island 
(Figure 2). 



produce any Manila clams. In 1979, 12 live and 32 dead clams 
were found in three isolated locations in this area (Port 
Harvey, Fife Sound, and Booker Lagoon). Live clams ranged 



In 30 years, Manila clams spread throughout the Strait in length from 31 to 55 mm and in age from 5 to 10 years. 



50 



BOURNE 



5 







15 



10 



5 







y 30 





UJ 




UJ25 
a: 

u. 



20 



15 



10 



5 







GALE 




MILBANKE 
SOUND 



Manila dams probably arrived in the Queen Charlotte is required to transport larvae a distance of approximately 
Strait-Alert Bay area in the late 1960's and again occupied 120 km across Queen Charlotte Sound. Assuming a mixed- 
the mid-portion of the intertidal beach. Populations are surface layer depth of 5 m, it would require a net onshore 
scattered and sparse, and apparently no recent reproduction Ekman transport of about 270 metric tons (t)/sec/km. 
or reintroductions have occurred. Whether the population Mean monthly values of this magnitude have been reported, 
is large enough to establish a reproducing population or if particularly in September at latitude 50°N and longitude 
continuing reintroduction of larvae must occur to maintain 130 W (Ballantyne and Wickett 1978, Table 27). 
it, is unknown. 

In 1980, a survey was made of beaches around Bella 
Bella in the central coastal area; Manila clams were found in 
substantial numbers on 14 of 46 beaches surveyed, again 
occupying the mid-to-upper portion of the intertidal beach. 
Densities ranged from 1 to 162 clams/m^ with the higliest 
occurring in Milbanke Sound and Gunboat Passage. Popula- 
tions were found on beaches close to the eastern edge of 
Hecate Strait and also on beaches well into some of the 
passages where residual surface current flow is outward into 
Hecate Strait. Shell lengths ranged from 11 to 56 mm and 
most clams were smaller than the commercial legal size 
limit of 38-mm shell length (Figure 3). Some samples had 
more than one year-class indicating the population is either 
consistently being supplied with larvae from other sources 
or, more likely, that local reproduction has occurred. 

Age analysis (see below) indicates the large dominant 
year-class of clams at Gunboat Passage is approaching 
4 years of age. Tlie two year-classes in Milbanke Sound 
are approaching 2 and 4 years of age and those in Gale 
Passage are 2 to 4 years in age. 

In 1981, another survey was carried out in the northern 
area, Principe Channel to Prince Rupert (lat. 53°10'N to 
54°20 N), but no Manila clams were found. 

The most northerly record of Manila clams in British 
Columbia is Rescue Bay, Mathieson Channel (lat. 52 30 N). 

DISPERSAL ROUTES 

The few clams found at Spider Anchorage in 1972 and 
1973 were probably the result of settlement of larvae from 
a spawning in the Quatsino Sound area that drifted across 
Queen Charlotte Sound (Figure 2). Water temperatures in 
the Quatsino Sound area, as recorded at Kains Island at 
the entrance to the Sound (Hollister and Sandes 1972), are 
sufficient to permit gametogenesis and spawning, particularly 
in years of above-average water temperatures which occurred 
in 1957, 1958, 1963, and 1967 (A. J. Dodimead, Pacific 
Biological Station, personal communication). Evidence in 
support of the transport of larvae across Queen Charlotte 
Sound is available from an examination of the zonal com- 
ponent of the Ekman transport (wind-driven surface flow). 
This transport is generally offshore during May to Septem- 
ber off northern Vancouver Island but there are years in 
which the net surface transport is onshore and relatively 

;i in August and September, at a time when sea surface 
temperatures are usually favorable for larval survival 
(Dodimead 1980). For a 3- to 4-week larval period (Yoshida 
1953, Williams 1978), a net movement of about 4.6 km/day 





LENGTH (mm) 



Figure 3. Length frequency distribution of Manila clams in the 
central coastal area of British Columbia, 1980. 



Manila Clams in British Columbia 



51 



Manila clam populations at Bella Bella in the central Manila clams occur at Port Harvey but not in Port Neville 

coastal area may have originated from more than one source, (Bourne, unpublished) which is about 17 km to the south 

Adult clams may have been planted by unknown individuals, of Port Harvey (i.e., towards Seymour Narrows) in John- 

but this seems unlikely because the clams occur over a wide stone Strait. If dispersal had been north througli the 

area suggesting a more general introduction. Transplants of Seymour Narrows-Yuculta Rapids area, one would expect 

Pacific seed oysters were made in 1967 to several locations the clams to populate beaches in the Port Neville area 

along the British Columbia coast in raft culture experiments before those in the Port Harvey area. 



(Quayle 1971). One location was Pruth Bay, Calvert Island, 
about 25 km south of Spider Anchorage, It is possible that 
Manila clams were transported to Pruth Bay along with the 
seed oysters and spawnings from these clams produced 
larvae which drifted northward up the coast. However, 
surveys in the Pruth Bay area in 1970 failed to find any 
clams and surface currents do not favor this route of dis- 
persal. Clam populations at Bella Bella may have originated 
in the Quatsino Sound area, or from spawnings at Spider 
Anchorage, or areas in between because water temperatures, 
currents, and the larval period are suitable for transporting if any local reproduction has occurred. Water temperatures 



The most plausible route for the introduction into the 
Alert Bay area would be from spawnings at Quatsino Sound. 
Current patterns (Thompson and Van Cleve 1936, Dodi- 
mead and Hollister 1962, Dodimead 1980) show larvae 
could be transported to the mainland side of Queen 
Charlotte Strait and then into the Alert Bay area. To date, 
Manila clams have been found only near the mainland side 
of Queen Charlotte Strait and not on the Vancouver Island 
side, which supports this hypothesis. 

The size structure of this population indicates that little 



larvae there from any of the three sources. However, unless 




ister and Sandes 1972) are too low to permit wide- 



the Spider Anchorage population is much more extensive spread reproduction, but local reproduction may occur in 
than recorded in 1973, it is likely that the central coast embayments such as Port Harvey or Booker Lagoon where 



population originated from the Quatsino Sound area because 
too few larvae would be produced from the sparse popula- 
tion at Spider Anchorage. 

The population structure of Manila clams in the Bella 
Bella area (Figure 3) shows it is either continually receiving 
(virtually yearly) introductions of larvae from southern areas 
or, more likely, successful local reproduction is occurring. 
Water temperatures at Mclnnes and Ivory islands (Dodimead 
1980) are sufficient for reproduction, particularly in years 
with above-average summer water temperatures. Water 
temperatures in protected local areas with restricted circula- 
tion would be higlier than at monitoring stations. Annual 
reproduction may not occur in years of low water tempera- 
tures, but the population may obtain sporadic spatfall from 
other areas such as Quatsino Sound. Expansion of the 
population may depend not only on summer water tempera- 
tures, but also on the severity of subsequent winters. Williams 
(1978) reported that clams which set in Puget Sound in 
September overwintered at under 2-mm shell length and 
were located in the top few centimeters of sediment. 
Because Manila clams inhabit a higli intertidal beach position, 
they could suffer mortalities during harsh winters. 



water circulation is restricted and water temperatures 
higher. Larvae from such local spawnings could populate 
other beaches in the Alert Bay area but surveys show that 
has not been extensive. 



REPRODUCTIVE CYCLE 

Ohba (1959) reported that Manila clams spawned twice 
a year in different parts of Japan, usually late spring and 
again in early-to-late fall. Yamamoto and Iwata( 1956) found 
a single spawning period which lasted from July to September 
in Hokkaido. Holland and Chew (1974) reported an 
extended spawning period for clams in Puget Sound; 
spawning began in late June and continued at intermittent 
periods during the summer and into autumn. Williams 
(1978) confirmed the extended spawning season in Puget 
Sound and observed two periods of settlement, a minor one 
in July and a major one in September. 

The reproductive cycle of Manila clams in the Strait of 
Georgia, as determined by examination of adults and analysis 
of larval development in the plankton, is similar to that in 
Puget Sound. Adults are ripe in early June and some spawning 
occurs in mid-to-late June because umbo larvae have been 



Introduction of Manila clams into the Queen Charlotte found in the plankton at the end of June; larvae have been 



Strait-Alert Bay area is less well explained. Adult clams may 
have been taken from the Strait of Georgia to the Alert Bay 
area via seed oyster transplants or by recreational clammers. 
However, this seems unlikely because Manila clams occur 
over a wide area which indicates a more general introduction. 
Another route would be north through Seymour 
Narrows-Yuculta Rapids region, but this also seems unlikely. 
If Manila clams had spread to the Alert Bay area via this 



found in the plankton until September. On the western 
coast of Vancouver Island, the reproductive cycle is probably 
similar to that in the Strait of Georgia, althougli spawning 
may be somewhat later because water temperatures are 
lower, particularly as one proceeds northward. 

Gonad samples were taken from the largest Manila 
clams from three beaches in the central coastal area in late 
May and early June 1980 to determine stages of gonadal 



route, one would have expected it to have occurred in the development. Gonads were preserved in Davidson's solution, 
late 1940's or early 1950's (particularly during a warm blocked in paraffin, sectioned at 5 pm, stained with 
summer such as 1958), rather than in the mid-1960's. haematoxylin-eosin, and examined microscopically. The 



52 



BOURNE 



five stages of the reproductive cycle described by Holland Columbia have distinct winter annul!. Tlie sample size from 



and Chew (1974) were used to classify development. The 
sample size was relatively small and only a single sample 



Alert Bay was limited but is included here for comparison. 
Von Bertalanffy (1938) growth parameters were calculated 



was taken, but the results are included here for information (Table 2), and the resultant curves drawn through the 



(Table 1). 



TABLE 1. 



Stages of gonadal development in Manila clams at three locations 

in the central coastal area of Britisti Columbia. 



Date 
1980 




Location 



Active 



Late 
Active Ripe 



Partially 
Spent Spent 



May 29 



May 30 



Leighton Island 
Milbanke Sound 

Horsefall Island 
Milbanke Sound 



June 3 Gunboat Passage 



OM 
OF 

OM 
OF 

OM 
OF 



12M 

4 F 

10 M 
6F 

7M 
3F 



M 

F 

M 

F 

M 

4 F 



OM 
5 F 

OM 
IF 

OM 
IF 



OM 
4F 

OM 

1 F 

OM 

OF 



No clams were in the early active stage. All males and 



points on Figure 4. 

Growth of Manila clams, as shown by k, was fastest in 
the Strait of Georgia, followed by the western coast of 
Vancouver Island; growth was slower in the central coastal 
area and slowest in the Alert Bay area. Calculating growth 
using the method of Gallucci and Quinn (1979) shows a 
higlrer omega value for clams from the Strait of Georgia, 
followed in order by those from the western coast of 
Vancouver Island, the central coastal area, and the Alert Bay 
area. Manila clams attain legal commercial shell length of 
38 mm in about 3.5 years in the Strait of Georgia; in about 

4 years on the western coast of Vancouver Island; in about 

5 years in the central coastal areas; and in 5.5 years in the 
Alert Bay area. Growth is fastest where water temperatures 
are highest (i.e., the Strait of Georgia), and slowest where 
water temperatures are lowest (i.e., the Alert Bay area). 

The Loo of the Von Bertalanffy equation shows that 



FUTURE DISPERSAL 



most females were in the late active stage. Five females although Manila clams in the central coastal and Alert Bay 

were spent; they probably spawned the previous summer areas have slower growth rates than those in the Strait of 

and had not begun to regenerate gonadal tissue in 1980. Georgia, they can attain a larger size which is similar to 

Seven females were partially spent; again, these clams what Weymouth and McMillan (1930) observed in razor 

probably did not completely spawn out during the previous clam populations along the western coast of North America. 

summer and had not begun to regenerate gonadal tissue in 

1980. Four females from Gunboat Passage were ripe. It 

is unlikely these clams developed to the ripe stage in 1980, No physical barriers exist to deter future expansion of 

rather they were probably ripe in 1979, did not spawn, and Manila clam populations in British Columbia; dispersal 

remained in a ripe condition througliout the winter and 

spring. This condition has been observed frequently in the 

butter clam Saxidomus giganteus (Deshayes) in British 

Columbia. 

Monthly mean surface water temperatures at Mclnnes 
and Ivory islands (Hollister and Sandes 1972, Dodimead 




depend on environmental parameters. Low water 
temperatures, slow growth rates, and cold winter tempera- 
tures will probably deter development of extensive Manila 
clam populations in the Queen Charlotte Strait-Alert Bay 
area. In the central coastal area, reproduction has apparently 
occurred. Surface water currents there drift northward 
1980) are about 10°C in May. Holland and Chew (1974) along the eastern side of Hecate Strait (Dodimead and 
observed that Manila clams in Puget Sound were in the late Hollister 1962) and could carry larvae northward. This was 
active stage in mid-April when water temperatures were the dispersal route of another exotic, Af>^flare/zana Linnaeus, 
about 8 C, hence development to the late active Stage at the which spread northward into Alaska and across Dixon 
end of May at Bella Bella is not unexpected. Spawning would Entrance into the Queen Charlotte Islands (Quayle 1964). 



probably not occur until August in years with normal or 
above-normal temperatures when the time-temperature 
period for gametogenesis is sufficient and water temperatures 



However, monthly mean surface water temperatures at 
Bonilla and Triple islands rarely attain 14 C which may be 
too low to permit gametogenesis, spawning, and larval 



high enougli for spawning. In below-normal summers, com- development of Manila clams. The high intertidal beach 
plete gametogenesis and spawning probably do not occur. 



GROWTH 



position during harsh winters could also cause extensive 
mortalities of any spat that miglit settle there. Develop- 
ment of an extensive clam population north of the central 
Growth was measured for Manila clams from various coastal area of British Columbia will probably be slow, 
locations in the Strait of Georgia, the western coast of The possibility exists that a race of Manila clams has 

Vancouver Island, Alert Bay, and central coastal areas, developed in these northern areas which undergoes gameto- 



Growth was determined by measuring shell length at genesis and spawns at colder water temperatures than 
winter annuli (straiglit-line distance between the anterior reported by Mann (1979). If this has happened, an extensive 



and posterior margins of annuli) with calipers to the nearest 
mm (Quayle and Bourne 1972). Most clams from British 



clam population could develop more quickly in this and the 
Alert Bay areas. 



Paiameteis of the 


MANILA Clams in British Columbia 

TABLE 2. 

von Bertalanffy growth curve, LX= Loo [ 1 - B exp {-K+)] , as obtained from measurements of slieU length 
at winter annuli of Manfla clams from various locations in British Columbia. 


53 


Location 


k 


+ tXSE L • +tXSE to 


±tXSE 


CO 


STRAIT OF GEORGIA 

Von Uonop Inlet 
Savary Island 
KuUeet Bay 


0.22 
0.30 
0.39 


0.061 67.7 9.4 -0.02 
0.053 55.1 3.4 +0.09 
0.025 47.6 1.0 +0.02 


0.191 
0.272 
0.051 


14.89 
15.98 

18.56 



WEST COAST OF VANCOUVER ISLAND 



Atleo River 


0.23 


0.072 


62.5 


8.3 


+0.01 


0.304 


14.38 


Hesquiat Harbor 


0.29 


0.172 


56.1 


13.8 


+0.002 


0.445 


15.71 


Toquart Bay 


0.31 


0.104 


49.1 


7.8 


-0.14 


0.187 


15.22 


Hiliiers Island 


0.26 




60.6 




-0.18 




15.76 


CENTRAL COAST 
















Horsefall Island 


0.15 


0.116 


75.1 


34.5 


+0.02 


0.416 


11.25 


Gunboat Passage 


0.14 


0.392 


84.5 


159.3 


+0.04 


0.111 


11.83 


Lady Trutch Island 


0.17 


0.075 


69.5 


15.5 


0.04 


0.389 


11.82 


Bella Bella combined 


0.15 


0.044 


73.0 


11.1 


-0.02 


0.239 


10.95 


ALERT BAY 
















Alert Bay combined 


0.14 


0.062 


68.8 


14.3 


0.28 


0.522 


9.63 



80.0- 



I 60.0- 





e> 




UJ 



40,0- 




20.0- 




1. 

2. 

3. 
4. 
5. 
6. 
7. 
8. 



BELLA BELLA 
ALERT BAY 
VON DONOP IN 
HILLIERSIS. 
ATLEO RIVER 
REFUGE COVE 
SAVARY IS. 
KULLEETBAY 



Spread throughout the Strait of Georgia and along the 
western coast of Vancouver Island where they are now 
abundant. 

Transplants of Manila clams to two northern areas of 
the Province were not successful. 

In the late 1960's and early 1970's, Manila clams spread 
to the Queen Charlotte-Alert Bay and central coastal areas. 
A reproducing population of clams is now established in 
the central coastal area. 

The reproductive cycle of Manila clams in the Strait of 
Georgia is similar to that in Puget Sound; clams spawn from 
mid-to-late June until September. A single sample from 
the central coastal area shows that reproduction can occur, 
particularly in warmer years, and spawning probably occurs 
in August. 

Growth of Manila clams is fastest in the Strait of Georgia 
and slowest in the Alert Bay area. 

Future dispersal northward is possible but will depend 
on environmental parameters such as water temperatures, 
currents, and severity of winters after spatfall. Development 
of a race of Manila clams that spawn at lower temperatures 
would hasten dispersal northward. 







5 



1 — 

10 



T — 

15 



ACKNOWLEDGM ENTS 



AGE (YEARS) 



Sincere appreciation is expressed to Dr. D. B. Quayle 



Figure 4. Predicted length at age Von Bertalanffy curves for Manila 
clams from several locations in British Columbia. 

SUMMARY 



for determinations of gametogenesis, 



of unpublished 



material, and review of the manuscript. I am also indebted 
to Mr. A. J. Dodimead for frequent consultations on 
oceanographic details and use of unpublished data. Ms. S. 
Manila clams were first found in British Columbia at Farlinger assisted with the field work and in the analysis of 
Ladysmith Harbour in 1936 and, within 30 years, had the growth data. 



54 



BOURNE 



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